1. Field of the Invention
The present invention relates to a vibration isolator for use in a vehicle, a general industrial machine or the like and adapted to absorb and attenuate vibrations from a vibration-generating portion thereof.
2. Description of the Related Art
In an automobile engine, a vibration isolator serving as an engine mount is interposed between the engine and the chassis, so as to prevent the vibrations of the engine from being imparted to the chassis. Vibrations of the engine include low-frequency vibrations such as shaking vibrations, as well as high-frequency vibrations such as idling vibrations, and vibration isolators of a liquid-sealed type have been proposed to absorb and attenuate the vibrations of both high and low frequencies.
The vibration isolator of the liquid-sealed type is provided with a pressure-receiving liquid chamber (a main liquid chamber) and an auxiliary liquid chamber, which are filled with a liquid and communicate with each other through two passageways. The sizes (cross-sectional areas and lengths) of these passageways are set for absorbing the shaking vibrations and the idling vibrations, respectively.
A valve is connected to the passageway for high frequencies, and this valve is closed during the occurrence of low-frequency vibrations so that the liquid does not flow through the passageway for high frequencies. Consequently, during the occurrence of the low-frequency vibrations, the liquid flows only through the passageway for low frequencies, thereby making it possible to effectively absorb the low-frequency vibrations.
An actuator such as a motor is used for opening and closing the valve for opening and closing the passageway. However, in order to open and close the valve, it is necessary to precisely control the rotating direction of a motor shaft and the rotational speed or rotational angle thereof. As a result, a control circuit becomes complicated, and a sensor or the like for detecting the opening and closing of the valve is required. In addition, to ensure the efficiency in assembling the vibration isolator and the reliability of an electric system, the actuator such as the motor is mounted on an outer surface of a body of the vibration isolator. Consequently, the size of the body of the vibration isolator becomes large, and there are sometimes restrictions in the space where the vibration isolator is installed. In addition, the control circuit is complicated, and the cost of the motor itself is high, so that there is a problem in that the cost of the vibration isolator as a whole becomes high.
In addition, although a method has been proposed for opening and closing the valve by mounting as an actuator a cylinder means on the outer side of the body of the vibration isolator instead of the motor, the size of the body of the vibration isolator becomes unavoidably large as in the case where the motor is used.